Liquid crystal panels intended for flat panel displays (FPDs) are manufactured by using exposure apparatuses. In an exposure apparatus, an image of a pattern drawn on a mask is projected onto a glass substrate to which photoresist has been applied, and the glass substrate is exposed to light. Such an exposure apparatus is disclosed by PTL 1. FIG. 9 is a schematic diagram of an exposure apparatus according to a related art. The exposure apparatus includes an illumination optical system IL that illuminates a mask 18, and a projection optical system that projects a pattern drawn on the mask 18 onto a substrate 14. The projection optical system includes polygonal optical member 1 having a first reflecting surface 1a and a second reflecting surface 1b, a concave mirror 17 having a first concave reflecting surface 17a and a second concave reflecting surface 17b, and a convex mirror 2. Furthermore, the projection optical system includes a lens barrel 3 that houses the above mirrors. Light emitted from the illumination optical system IL in the −z direction is transmitted through the mask 18 and is redirected in the +y direction by the first reflecting surface 1a of the polygonal optical member 1, which is provided below the mask 18. The light redirected by the first reflecting surface 1a of the polygonal optical member 1 is reflected by the first concave reflecting surface 17a, the convex mirror 2, the second concave reflecting surface 17b, and the second reflecting surface 1b in that order and falls onto the substrate 14.
The convex mirror 2 is provided with a supporting member 15 that supports the convex mirror 2. FIG. 10 is a sectional view of the projection optical system taken along an x-y plane. The supporting member 15 is a beam-like member provided between the polygonal optical member 1 and the concave mirror 17 and extending in the x direction that is perpendicular to the optical axes of the convex mirror 2 and the concave mirror 17. The supporting member 15 supports a side of the convex mirror 2 that is opposite a reflecting surface of the convex mirror 2. Two ends of the supporting member 15 are supported by respective supporting surfaces of the lens barrel 3.
The illumination optical system IL emits a beam of illuminating light whose cross section has an arc shape with a predetermined width, and illuminates the mask 18 within an arc-shaped illuminating area. Therefore, the areas of the first reflecting surface 1a, the first concave reflecting surface 17a, the convex mirror 2, the second concave reflecting surface 17b, and the second reflecting surface 1b that are to be illuminated each also have an arc shape of a predetermined size. FIG. 11 is a plan view of the projection optical system that is seen from the side of the concave mirror 17. As illustrated in FIG. 11, the convex mirror 2 is surrounded by arc-shaped beam areas 16. The supporting member 15 is positioned between the beam area 16 on the upper side and the beam area 16 on the lower side.
If the area of the mask 18 that is to be illuminated is widened or the incident angle of the light to be taken into the projection optical system is increased for the purpose of improving the resolution or the throughput, the path of the light beam in the projection optical system is also widened. The path of the light beam in that case is illustrated by dotted lines in FIG. 9. As illustrated by the dotted lines in FIG. 9, the light beam reflected by the polygonal optical member 1 overlaps the supporting member 15 that supports the convex mirror 2, and the overlapped portion of the light beam is blocked or reflected by an upper portion A of the supporting member 15. Consequently, favorable imaging performance cannot be provided.
To avoid such an eclipse of the light beam by the supporting member 15, the size of the projection optical system, including the sizes of the polygonal optical member 1, the convex mirror 2, and the lens barrel 3, may be increased. However, the increase in the size of the projection optical system leads to an increase in manufacturing costs and an increase in the installation space. Moreover, the sizes of the polygonal optical member 1 and the convex mirror 2 increase, and the weights of the polygonal optical member 1 and the convex mirror 2 increase correspondingly. Hence, the eigenvalues (natural frequencies) of the polygonal optical member 1 and the convex mirror 2 are lowered. If the eigenvalues are lowered, the amplitude of vibration due to disturbance, if any, becomes large. Accordingly, the change in the position of the image plane of the projection optical system increases, the imaging performance in the exposure process performed on the substrate changes periodically and significantly, and the pattern projected on the substrate is distorted. Consequently, the resulting pattern formed on the substrate may have defects or include lines whose widths are nonuniform.